In many communication applications full duplex transmit/receive (TX/RX) operation is required. One such application is in the implementation of the CDMA communication standard. A more specific application is in the area of wireless handsets and PCM/CIA cards in computers, which implement the CDMA standard.
Often one desires to implement a multi-band communication system. A typical example is a dual band wireless handset where the two bands implemented are the U.S. cellular (typically 800-1000 MHz) and U.S. PCS (typically 1800-2000 MHz). A current radio frequency (RF) realization of such architecture employs entire, separate TX and RX chains for the RF. This realization is driven by the rather wide separation of the two bands along with the fact that most of the components used are by necessity narrow band devices. Specifically, these narrow band devices include, but are not limited to, band pass filters (BPF""s), power amplifiers (PA""s) and duplexers used for either band.
Basic building blocks of present day dual band wireless CDMA handsets are BPF""s and duplexers based on electromagnetic (em) resonators. Examples are short circuit monoblock and stripline filters, which support transverse electromagnetic (TEM) waves. They have an added advantage in that they""re physically short as well as being relatively simple structures to fabricate. Others are surface acoustic wave (SAW) or film bulk acoustic resonator (FBAR) technologies. The relative merits and limitations of SAW and FBAR devices are well known to those skilled in the art. In all cases (TEM or acoustic resonator) the resultant filters or duplexers must meet tight specifications in their designated bands. Going to multiple bands conventionally requires using multiplexers in place of duplexers or multiple duplexers with multiplexers following them or the use of switches to select desired bands. Any of these options take up precious board space, add cost and complexity to the system.
The advantages of using ferro-electric (f-e) based components to design and fabricate low order, high performance filters have been outlined in U.S. patent application Ser. Nos. 09/904,631; 09/912,753; and 09/927,732. F-E tuning is limited, however, to a relatively narrow band around a desired operating frequency if the lowest possible insertion loss (I.L.) is desired. The requirement for minimum I.L. in wireless CDMA systems has also been discussed previously and will not be repeated here. F-e tuning as it applies in this application will be directed to its use with em resonators, such as, for example, monoblock, stripline and coaxial resonator-based devices. As is known to those skilled in the art, lumped elements (discrete inductors and capacitors) can be used at appropriate lower frequencies or at higher frequences where the added loss inherent to lumped element components can be tolerated.
The need to use two distinct RF architectures to realize two distinct bands is unfortunate, as it significantly increases cost and pwb board area required to realize the design. The need to specify and carry two sets of components further adds to cost and overhead. Clearly, if one wanted to realize say three, or more bands, the problems of increased cost, size and signal interference would make the attempted realization prohibitive, especially in high volume.
It would be advantageous if one could design a dual band (or higher) system with one set of components rather than using two (or more) sets.
Wireless communication systems require highly selective frequency filtering of wireless communications signals. Filters occupy a significant percentage of wireless communication device space. Additionally, the process of filtering a signal results in the attenuation of the desired signal. That is, power is consumed by filters in the process of selecting the correct frequency band for transmission or reception. This power consumption is known as insertion loss (I.L.). This results in increased power consumption in the case of transmission and decreased sensitivity in the case of reception. This results in decreased talk time and a possibly degraded communication link.
Also, wireless communications devices commonly operate in multiple frequency bands. Typically, different filters are required for each frequency band. Thus, multiple filters consume even more space in a wireless communication device. Finally, filters comprise a significant portion of the cost of a wireless communication device.
Accordingly, a novel filter is provided with excellent performance in terms of out-of-band rejection and in band insertion loss. The filter is tunable over a range of frequencies and switchable over a broader range of frequencies. Preferably, the filter is smaller than those known in the prior art. Thus, a single filter can be used for multiple frequency bands, replacing multiple filters in a prior art system.
Thus, the filter accomplishes the filtering of wireless communication signals, while occupying less space than previous filters. Also, the filter costs less than previously known filters. Additionally, talk times and standby times are increased over wireless communication devices operating with previously known filters.
One technique for doing this involves the design of dual band BPF""s and duplexers based on micro-electro mechanical system (MEMS) switches used in conjunction with f-e tunable low order, narrow band BPF""s. This approach reduces the size and cost of the required parts by eliminating one (or more) set(s) of components. Using a similar approach for PA""s and LNA""s allows their use in covering two (or more) widely separated bands in frequency. Band switchable power amplifiers and lower noise amplifiers are discussed in U.S. patent application Ser. Nos. 10/075,727, 10/076,171, and 10/075,507. Thus, one may realize a multi-band electronic communication system without the need to introduce redundant parts or electronic switches.